1) Field of the Invention
The present invention relates to friction welding and, more specifically, to an apparatus and method for friction stir welding with a pin having a variable rotational speed.
2) Description of Related Art
Friction stir welding is a process in which a rotating tool, such as a pin or probe, is urged into and/or through a workpiece, e.g., to join multiple members of the workpiece in a solid state or to repair cracks in a workpiece. Typically, the pin extends from a shoulder, which can be flat, concave, or otherwise contoured, and the shoulder is urged against the workpiece so that the pin is urged therein. The pin is then urged through the workpiece to form a continuous weld joint. For example, during one conventional friction stir welding process, the probe of the rotating tool is plunged into a workpiece or between two workpieces by a friction stir welding machine to produce the required resistance force to generate sufficient frictional heating to form a region of plasticized material. The tool can be tilted approximately 3° relative to the workpiece such that the trailing edge of the shoulder is thrust into and consolidates the plasticized material. Upon solidification of the plasticized material, the members of the workpiece are joined along the weld joint. The magnitude of force exerted by the friction stir welding tool must be maintained above a prescribed minimum in order to generate the required frictional heating. Friction stir welding is further described in U.S. Pat. No. 5,460,317 to Thomas et al., the contents of which are incorporated herein by reference.
Some friction stir welding pins define helical threads, which can be right-hand threads, left-hand threads, or a combination thereof. Generally, the threads increase the mixing of the plasticized material in the region of the weld joint. Further, a support device is often provided behind the weld joint, e.g., an anvil or other backing device provided opposite the shoulder, to prevent deformation of the workpiece by the forces exerted by the friction stir welding tool and maintain dimensional tolerances. The shoulder and the backing device are typically urged together during welding to prevent material that is plasticized by the friction heat generated by the welding tool from flowing out of the weld joint. The inward clamping of the shoulder and backing device also consolidates the joint to minimize porosity and provide a weld joint having the desired surface finish.
A welding machine that is used to move the friction stir welding tool through the workpiece must often exert significant forces on the welding tool to move the welding tool at a particular speed through the workpiece. The force can depend in part on the thickness of the workpiece, the material of the workpiece, the length and diameter of the pin of the tool, the rotational speed of the tool, the speed of the tool through the workpiece, and the like. The forces required to achieve a particular weld joint can limit the speed at which the machine can be operated and, hence, the speed at which the joint can be formed. Further, the machine and tool typically require periodic maintenance and/or replacement, especially where the forces on the machine and the tool are significant.
Thus, conventional friction stir welding devices have been shown to form weld joints in which the material of the workpiece is plasticized and mixed, and the resulting granular structure in the weld joint is refined. However, a need continues to exist for an improved friction stir welding tool for forming weld joints in which the material has been optimally mixed and refined. The tool should allow for friction stir welding of a variety of workpieces, including thick workpieces, using a variety of welding machines and tools. The tool should be capable of effectively plasticizing and mixing the material. Preferably, the tool should also be compatible with features and methods for reducing the forces on the tool and machine during welding.